We have investigated some of the cell sources, biological activities and biochemical characteristics of interleukin (IL 1). We have established that a subpopulation of OKM1+, DR+, B73.1+ large granular lymphocytes (LGL) with natural killer (NK) activity when stimulated by endotoxin produce IL 1 and can also act as accessory antigen-presenting cells (APC) that have the capacity to activate T lymphocytes. In contrast another subset of LGL (DR-, OKM1-) can be stimulated by lectins to produce lymphokines such as IL 2 and interferon. In addition, a number of EBV-transformed human B cell line cells were also demonstrated to produce IL 1 and to have APC capabilities. Several of these B cell lines, in addition, spontaneously produced factors that inhibit effects of IL 1, which we have termed "contra IL 1". Human IL 1 was purified to homogeneity by a sequence of chromatography techniques. Low doses of purified IL 1 were shown to stimulate murine mammary epithelial cells to secrete collagen type IV, a characteristic constituent of basement membranes. In addition, such IL 1 promoted in vitro tumoricidal effects of human monocytes on melanoma tumor cells. This effect of IL 1 could be blocked by indomethacin and emulated by prostaglandin E1 and E2 and dbcAMP. These observations suggest that IL 1 may have autocrine functions and through its effect in inducing or maintaining monocyte tumoricidal capabilities IL 1 may participate in host defenses against tumors.